Zonation and geostrophic flow in Drake Passage

Includes bibliographical references (leaves 74-76)The Antarctic Circumpolar Current in Drake Passage comprises four regimes which are characterized by distinct temperature-salinity relationships in the near-surface waters and by different depths of common temperature-salinity characteristics in deeper water. Between the Subantarctic Zone in the northern Passage and the Antarctic Zone father south is a transition region, the Polar Frontal Zone. The southernmost regime is the Continental Zone which is restricted to a narrow band near the southern continental slope. Historical hydrographic data are used to determine the mean positions of the fronts which separate the zones. Expendable bathythermograph data show that the two northern fronts, the Subantarctic Front and the Polar Front, fluctuate about 100 km north and south of their mean positions. No seasonal differences in the mean frontal positions are evident. Baroclinic geostrophic transport calculations relative to 2500 db are made from synoptic hydrographic station pairs which span the Passage. The range of all such transport estimates is large when the positions of fronts relative to the station positions are not considered. .The variability of the estimates is reduced to about 20% of the mean transport when calculations are made for station pairs which encompass the Subantarctic and Continental Zones. The mean summer transport of 78.7 x 10^6 m^3 s^-1 is about 10% higher than the winter mean of 70.6 x 10^6 m^3 s^-1. Transport calculations are also made from the average values of dynamic height and transport function within the Subantarctic Zone and the Continental Zone. The mean summer transport of 80.9 x 10^6 m^3 s^-1 is significantly different from the winter mean of 73.4 x 10^6 m^3 s^-1. The largest flow in summer is between the Subantarctic and Polar Frontal Zones, whereas in winter the flow between the Polar Frontal Zone and the Antarctic Zone is predominant. The lower winter transport is associated with a shallower depth of the isopycnals throughout the water column in the Subantarctic Zone..